Electromagnetic wave absorbers and anechoic chambers are in widespread use throughout the world in making antenna and reflectivity measurements. Chambers have been designed as general purpose facilities in order to achieve free space conditions for measuring the characteristics and properties of various components and systems, and are employed for a wide variety of measurements; others have been designed for particular types of measurements such as antenna impedance, gain, beamwidth, circularity, cross polarized component levels, antenna patterns, monostatic and bistatic radar cross-section patterns, system sensitivity, system susceptibility, system compatibility, effective radiated power, boresite alignment, radome error tracking error, etc.
Anechoic chambers provide a standard, reproducible environment which minimizes interfering energy disturbances for the measurement of a wide variety of electrical and electronic devices in order to establish or confirm that they meet certain requirements, such as spurious, harmonic, and noise emissions. However, conventional absorbers, used to cover the walls of indoor ranges for antenna and scattering measurements, for example, limit the performance of these ranges. In order to achieve the desired low reflectivity properties, good absorbers must provide the following two characteristics:
1. a smooth transition from air into the absorber, and PA1 2. complete absorption of the wave inside the absorber. absorber.
Presently available absorber materials typically use homogeneous material cut into either wedges, square pyramids, or cones. The pyramidal-shaped or cone-shaped elements are arranged in the chamber so that they project inwardly into the interior thereof. The absorbers are formed of a low density material that exhibits low dielectric properties and which is coated or impregnated with a substance that inherently absorbs microwave energy. As the microwave energy impinges against the tapered surfaces of the geometrically shaped absorber, part of the energy penetrates into the absorber panel while a part of the energy is reflected. Because of the configuration of the absorber panel, most of the reflected energy is reflected in a direction toward another absorbing surface of the panel rather than being reflected back into the interior of the chamber. Pyramidal and wedge absorbers usually provide better reflectivity performance at microwave frequencies than do flat or planar layers made from multi-layers.
Typically, the component to be evaluated in an anechoic chamber is placed at one end of the chamber facing toward a position at the opposite end from which a microwave energy signal can be beamed toward the device under observation. Although the signal is beamed directly at the device being observed, as the signal leaves the source of energy illumination the energy waves tend to diverge to form a signal of constantly expanding cross-section. The microwave energy absorbing material which lines the side walls, floor and ceiling of the chamber is intended to absorb microwave energy which strays too far from the axis of the signal beam and impinges against these surfaces of the chamber. Ideally, all microwave energy impinged against the absorber material is absorbed so that no wave energy is reflected back into the interior of the chamber to cause interference with the signal beam and inaccuracies in the measurements being taken.
The achievement of the lowest possible level of reflected energy in an anechoic chamber depends upon the proper manipulation of two variables: (a) the characteristics of the absorbing materials used to cover the internal chamber surfaces; and (b) the shaping of the chamber to direct residual reflected energy away from the quiet zone or working volume.
Compact range technology has been significantly improved by using new reflector designs and pulsed radars with large dynamic ranges. However, the scattering from the absorber-coated walls was found to be a serious limitation on compact range performance. Accordingly, several studies were made in an effort to improve absorber performance. Multiple layer wedges were designed (FIG. 1) such that the complex dielectric constant progressively increased from the outer layer through the inner layers. This approach gave better reflectivity than commercially available homogeneous wedge designs because it provided a more gradual transition from air into the absorber. Complete absorption can be achieved by using internal layers with sufficiently high loss. However, the multi-layer approach has the disadvantage that numerous layers are needed to achieve extremely low reflectivity levels.